Internal combustion (IC) engines for vehicles have several moving components which require lubrication. These include rotating shafts, sliding pistons etc. Lubrication occurs by the presence of oil. Oil is usually pumped around the engine by an oil pump. The oil pump will pick up low pressure oil from a sump, and pressurise it before delivery to the engine. Various pressure drops occur as the oil passes through the engine, and the oil eventually returns to the sump for recirculation.
The pumping effort required by the oil pump is determined by many factors. Some factors are inherent in the design of the engine (e.g. clearances and the path through which the oil must pass) and some factors vary through the operating cycle of the engine itself. For example, pumping effort decreases with a decrease in the viscosity of the oil, which in turn decreases as the engine (and oil) warms up. Therefore, it is generally much harder to pump oil around a cold engine because the cold oil has a high viscosity. Once the engine has warmed up, the pump does not have to use as much energy to pump the oil.
Various pump designs are available. Rotary positive displacement pumps such as gear pumps and gerotor pumps are common in this field, and are generally powered a drive connected to a pump input shaft. In some cases, the drive is the engine crankshaft (connected via a belt and pulley). In other cases, the drive is an electric motor.
Electrically driven oil pumps are increasingly common in modern engine design because they offer advanced control. Crankshaft driven pumps are dependent on engine speed, or require a gear train between the crank shaft and pump input shaft. The speed and fluid power output of electrically driven pumps can be varied more easily with electronic control. Electrically driven pumps also have fewer restrictions on placement of the pump (i.e. the input shaft of the pump does not need to be aligned with the crankshaft).
A problem with electrically driven oil pumps is the “cold start” condition. Because of the amount of pumping effort required to drive the cold oil through the engine, the electric motor need to produce a significant amount of torque (and therefore power). The intermittent need to produce a large amount of torque can reduce the life of the motor. Further, the cold start condition represents the “maximum power” design point for the electric motor driving the pump. In other words, the motor needs to be designed for this condition, but for most of the operation of the engine (when it is warm), the motor is not operating anywhere near capacity (i.e. it needs to produce less torque than the maximum power condition). Therefore, a much larger motor is usually provided than is necessary for most of the duty cycle. This increases cost and complexity, and takes up space in the engine.